The commercial building business continues to place a strong focus on Active Humidity Control (AHC) equipment for commercial air conditioning (AC) applications. This has grown out of an understanding that for building applications, the independent control of humidity and temperature afforded by AHC offers marked advantages over the traditional “cool first” approach. AHC conserves energy while affording building occupants immediate and lasting improvements in comfort, health and indoor-air quality. However, such AHC is not presently used in ships, though ships could also benefit from such systems.
Current AC systems aboard ship use the traditional “cool first” approach, in which the air in the ship's compartments is simultaneously dehumidified and cooled to prescribed environmental conditions. The demand on the cooling system is especially acute in a hot and humid marine environment where moisture levels in the compartment replenishment air delivered to the AC system are higher than those encountered on land. AC systems, therefore, are designed/rated for the abnormally high heat loads needed to accommodate these environmental conditions. Because the resulting systems are very large and severely taxed in producing enough chilled water to lower both the absolute humidity (moisture content) and temperature of the compartment air to the prescribed conditions, they consume a tremendous amount of the generated electrical power on the ship.
Traditional vapor-compression AC systems are designed to remove both sensible heat and latent heat by cooling the outside air below the dew point to condense out water vapor. A large amount of electricity is required to provide the additional chilled water required for this large latent heat load. Dynamic-desiccant-based AC systems, on the other hand, use a desiccant to remove moisture from the outside air prior to cooling the air with traditional chilled water. Some type of heat source then regenerates the desiccant.
Desiccants are a class of materials that have a great affinity for capturing and retaining water and are used in many applications where the presence of water or water vapor would be detrimental. Desiccants fall into two broad categories: solids and liquids. Liquids are usually absorbents, which means they undergo a physical or chemical change when they collect moisture. Sodium chloride, commercial table salt, absorbs moisture from humid air and eventually becomes crystallized a chemical and physical change. In contrast, solid desiccants are usually adsorbents, which means they collect water vapor on their surface but do not undergo a chemical or physical change. The low-cost granules used in pet litter boxes are alumina silicate clay, which is also a dry desiccant material. Silica gel is a typical dry adsorbent desiccant in which the crystals appear to have a smooth sealed surface, yet a microscope reveals a massive internal network of passages and crevices. Desiccant materials such as these that adsorb moisture from humid air can collect between 20 and 40% of their dry weight in water vapor.
There are two processes in which desiccants are used: dynamic desiccation and static desiccation. Dynamic desiccation is a continuous and cyclic process in which a desiccant material that has adsorbed moisture from a supply-air stream is subjected to a hot-air stream which dries out, and thereby regenerates, the desiccant for reuse in supply-air desiccation. Static desiccation also removes moisture from air, but there is no regeneration process. The desiccants are simply removed and discarded when saturated with moisture.
Static desiccants are primarily used in packaging, storage, and preservation of medicines, electronic and mechanical equipment, and other materials that have adverse or undesirable reactions to the presence of moisture. Dynamic desiccants are used in various building dehumidification applications, refrigeration systems, air-handling equipment, compressed-gas generation (air, nitrogen), vessel lay-up, reduction gears, etc. The primary desiccants used for these applications are molecular sieve, activated alumina, and anhydrous calcium or hygroscopic salts; these materials can be formed into granular beds or rotary wheels.
The wheel form of the dynamic desiccant is commonly known as the desiccant wheel and is used extensively in the design of desiccant-based, air-conditioning systems. The desiccant wheel is typically a circular device that is composed of a plurality of thin sheets of plastic or metal that are coated with a desiccant. The wheel, which is situated in the ducts of the air-conditioning system, is perforated to allow for the passage of air. The duct system is split to provide two countercurrent flow passages, one of which furnishes the supply air being dehumidified and the other which furnishes the air needed to regenerate the desiccant. The wheel slowly rotates to facilitate the transfer of moisture from the saturated desiccant to the regeneration air.
While supply air treated by dynamic desiccation has a moisture content much lower than that achieved by conventional chiller coils, commercial applications of the desiccation process generally do not exhibit high energy savings since a heat source, such as fossil-fuel-fired water or air heater or an electric heater, is required for indirect or direct heating of the regeneration air.